Measuring borehole temperatures employing diode junction means

ABSTRACT

Method and apparatus for measuring temperatures and temperature anomalies along the longitudinal axis of a borehole penetrating subterranean formations characterized by employing a constant current flowing through a diode junction means and measuring the variations in voltage across the junction as an indication of the temperature. The diode junction means is linear up to temperatures of about 350* F and linear enough to be usable to temperatures as high as 400* F. Signals are generated that are related by the voltage drop across the diode junction means to the temperature and transmitted to the surface where they are converted to an analog indication of temperature. The analog indication of temperature is differentiated to emphasize temperature anomalies. Specific structure of the diode junction means and a temperature compensated constant current regulator is also disclosed.

United States Patent Stout [4 June 20, 1972 MEASURING BOREI'IOLETEMPERATURES EMPLOYING DIODE JUNCTION MEANS Beautord F. Stout,Grandview, Tex.

[72] Inventor:

[73] Assignee: Worth Well Surveys, Inc., Fort Worth,

Tex.

[22] Filed: Sept. 8, 1969 21 Appl.No.: 855,982

52 use: .13/154, 181/.5FS [51] IntJl lh49/00 [58] FieldolSearch..307/310;324/l;l81/.5; 73/154 [56] References Cited UNITED STATESPATENTS 2,676,489 4/1954 Basham ..73/342 3,330,158 7/1967Simonyanetal.... ...73/362 3,421,375 1/1969 Dimon ..73/362 3,430,0772/1969 Bargen 307/310 3,496,286 1/1970 Schwartz ..73/362 OTHERPUBLICATIONS Scientific Instruments 10/70, Vol. 3, pg. 782- 784 AMultichannel Temperature Monitor" by Andersa.

Primary Examiner-Samuel Feinberg Assistant Examiner-N. MoskowitzAnomey-Wofford and Felsman [57] ABSTRACT Method and apparatus formeasuring temperatures and temperature anomalies along the longitudinalaxis of a borehole penetrating subterranean fonnations characterized byemploying a constant current flowing through a diode junction means andmeasuring the variations in voltage across the junction as an indicationof the temperature. The diode junction means is linear up totemperatures of about 350 F and linear enough to be usable totemperatures as high as 400 F. Signals are generated that are related bythe voltage drop across the diode junction means to the temperature andtransmitted to the surface where they are converted to an analogindication of temperature. The analog indication of temperature isdifferentiated to emphasize temperature anomalies. Specific structure ofthe diode junction means and a temperature compensated constant currentregulator is also disclosed.

MEASURING BOREHOLE TEMPERATURES EMPLOYING DIODE JUNCTION MEANSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to measuring temperature in boreholes penetrating subterraneanformations.

2. Description of the Prior Art It has long been recognized that aprofile of temperature along boreholes penetrating subterraneanformations could convey useful information to the petroleum engineer orgeologist. For example, expansion of gas from a gas zone or gas cap intoa wellbore effects a cooler than normal temperature. Such temperatureanomaly may effect a reversal in the thermal gradient at one or morepoints along the wellbore. Thief zones along the borehole may markabrupt changes in the temperature. In measuring solely the absolutetemperature along the borehole, the changes may be so small as to beoverlooked since they will be only a very small percentage of the total.If, however, a differential of the temperature is taken, the temperatureanomalies are delineated with great clarity. The problem in the priorart approaches has been to obtain a temperature sensor that was linearover the range of temperature encountered in most subterraneanformations; for example, temperatures of up to 350 F.

It is known in the prior art to employ thermocouples to measure downholetemperatures. With thermocouples the different metals form a junctionthat generates a spontaneous EMF which is indicative of the temperature.Thermocouples have been demonstrated to be disadvantageous because ofthe very small EMFs generated and the difficulties in translating thetemperature into useful information at the surface.

It is known in the prior art to use two temperature sensors; such as,thermistors; spaced a distance apart and their output compared. Thisapproach has been useful for several years in delineating temperatureanomalies. This approach was not altogether satisfactory since itdepended upon two essentially non-linear sensors spaced at differentpoints in the wellbore.

More recently, a single temperature sensor was employed in the downholetool and the output of the sensor at one location stored in a computerand the difference between it and a subsequent reading at another depthcomputed. This approach was expensive, elaborate, and complex; and alsosuffered when the temperature sensor was non-linear.

The prior art devices ordinarily employed thermistors in practicalembodiments although therrnistors are notoriously non-linear. Sensistorswere tried. Although less non-linear than transistors, the sensistorshad positive temperature co-efficients and were more difficult to employin borehole tools at the elevated temperatures encountered insubterranean formations, and they were still non-linear to anappreciable degree.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic representationof apparatus embodying the invention being used to log temperaturesalong a borehole penetrating subterranean fonnations.

FIG. 2 is a partial electrical schematic diagram showing the temperaturesensor and the temperature-compensated constant current regulator usefulin one embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS It is a primary object of thisinvention to provide method and apparatus for measuring temperaturesalong the longitudinal axis of a borehole penetrating subterraneanformations employing a temperature sensor that has a negativetemperature coefficient and is accordingly readily useful; and that islinear over the range of temperatures encountered in most subterraneanformations.

Other specific objects will be apparent from the following descriptivematter taken in conjunction with the drawings.

Referring to FIG. 1, downhole tool 11 is suspended in borehole l3penetrating subterranean formations 15 by cable 17. Cable 17 passes overdepth measuring sheave 19 which drives a depth function generating means21. Cable 17 is spooled from or onto cable drum 23, and has conductormeans electrically connecting elements in the downhole tool viaconventional slip rings and brushes 25 with the remainder of the surfaceequipment.

Downhole tool 11 comprises respective sections housing temperaturesensor 27, constant current regulator 29, amplifier means 31, signalgenerating means 33, transmitter, or output, means 35, power supplies37, collar locator and amplifier 39, and cable head 41.

Temperature sensor 27 comprises diode junction means having at aconstant current flow a substantially linearly varying voltage drop overthe range of temperatures of 60350 F. While the diode junction meansmay'comprise a single diode junction, it is preferable to employ aplurality of such diode junctions 43, FIG. 2, connected in series toincrease the sensitivity of the temperature sensor thereby efiected and,consequently, reduce the requirements on the remainder of the electronicelements in the downhole tool. A particularly preferred temperaturesensor is formed by eight of the diode junctions created by seriallyconnecting two units of four diode junctions each. Diode junctionsformed in silicon diodes are excellent in providing the requisitelinearity between 60 and 350 F. In fact, they are linear enough to beuseful at temperatures as high as 400 F. Other diode junctions; such asthose formed in germanium or even in the compound semiconductormaterials; may be employed when they behave similarly. While thisinvention is not to be limited by theory, it appears that the diodejunction has available a greater number of electrons and holes forcarrying the constant current as the temperature increases,Consequently, there is a lower voltage drop across the junction atelevated temperatures. There is, in effect, a sort of an expansion ofthe junction also as the temperature increases. While difierent diodejunctions may have the desired linearly varying voltage characteristicsover certain ranges at different current levels, it has been found thatwith silicon diodes, a current level of about one-half milliampereafi'ords satisfactory results. The one-half milliampere is an excellentamount of current to break down the diode when it is forward biased, yetnot create other problems, such as overheating. Current levels may bevaried between a few hundredths of a milliampere to the temperature atwhich the diode begins to heat up; ordinarily, for example, about 60milliamperes.

Constant current regulator means 29 comprises a field effect transistor45, FIG. 2. Field effect transistor 45 has its gate lead 47 connectedwith one lead 49 which may be source or drain depending on polarity ofthe interconnection. Field effect transistor 45 has its other lead 51connected with respective diode junction means 43. To maintain the flowof current constant regardless of change in temperature as the downholetool 11 traverses borehole 13, a series connected resistor meanstypified by resistor 53 and a diode means typified by diode 55 areemployed in the circuit connected with intermediate lead 49 and thejuncture of gate lead 47 and power source 57. The resistor 53 and diode55 may be reversed and obtain the same result. Power source 57 isindicated as a battery although it may be a DC voltage tapped from anyappropriate source.

Although the embodiment illustrated in FIG. 2 forms a simple anddependable constant current regulator, any other constant currentregulator means that will maintain the current As illustrated in FIG. 2,conductor 59 connects the amplifier means with the juncture of the diodejunction means and the constant current regulator means. The amplifiermeans generates an output signal that is a function of the voltagepresent at this juncture. The amplifier means may be any amplifiercapable of generating an output signal that is uniquely representativeof the voltage present at the juncture of the diode junction means andthe constant current regulator means. For example, it has been foundthat an operational amplifier capable of effecting a voltage between apositive 15 volts and a negative 15 volts upon the change of the voltageon conductor 59 of from about a negative 5.6 volts down to about anegative 3 volts is satisfactory. The term operational amplifier is atenn of art and adequately describes the amplifier means for effectingthe desired results. Suitable operational amplifiers are commerciallyavailable. An operational amplifier that has been found satisfactory isthe Philbrick T-52. It effects output voltages beginning at about plus7.5 volts (v) at a voltage input of minus 5.6 v on conductor 59; growingmore negative at a ratio of about 9:1; and finally effecting about minus11 v output at a voltage input of about minus 3 v on conductor 59. Witha sufficient number of diode junctions in the diode junction means, theamplifier means may be omitted. For example, with about 80 diodejunctions serially connected in the diode junction means, the amplifiermeans can be omitted, and the tool remain operable with only a minormodification of the signal generating means, as described hereinafter.

The signal generating means 33 in the signal generating means section ofthe downhole tool is connected with the amplifier means forgeneratingsignals that are a function of the output signal from theamplifier means. Specifically, a unijunction transistor may be employedto oscillate at a frequency that increases as the voltage output signalgrows more negative. Thus, it will be seen that the frequency-typesignal from the unijunction transistor is a function of the temperaturebeing measured by temperature sensor 27. For example, one unijunctiontransistor found useful oscillates at an output of about five cycles persecond per degree F. This increasing frequency coupled with the initialthreshhold o'f oscillation produces about 2,000 cycles per second at atemperature of about 300 F. The unijunction transistor employed as thesignal generating means should preferably be linear over the ranges ofthe output signal, It has been found that a D5Kl or a 2N492 typeunijunction transistor will operate satisfactorily. These unijunctiontransistors are commercially availa ble. While the unijunctiontransistor has been described as the signal generating means andconverts the output signal of the amplifier means into a frequency-typesignal, any other signal generating means may be employed if it willeffect an output signal that can be transmitted to the surface and beuniquely related to a temperature measured in the borehole. If a largenumber of diode junctions are employed in the diode junction means, theamplifier means is omitted, and the same relationship of frequency andtemperature are employed, then the polarity impressed across the diodejunction means is reversed. On the other hand, the polarity may bepreserved and an inverse relationship of frequency and temperatureemployed such that the frequency decreases as the temperature increases.Where the amplifier means is omitted and the signal generating means isconnected directly to the diode juncn'on means, it may be advisable toalter the voltage level of operation of the signal generating means. Inthe example given hereinbefore, employing about 80 diode junctions inthe diode junction means, the upper voltage of operation of theunijunction transistor, comprising the signal generating means, might beabout a positive 50 volts and grow more negative with increasingtemperature to about a positive 30 volts, the voltage base beingmaintained by suitable means such as Zener diodes. The unijunctiontransistor would accordingly see about the same voltage change itpresently sees by way of the operational amplifier.

The transmitter means 35 in the output section of downhole tool 11 isconnected with the signal generating means for transmitting the signalto the above-ground equipment. It has been found that a satisfactorytransmitter means comprises a transfonner and two transistors. Theresulting transmitter means will satisfactorily transmit thefrequency-type signals to the above-ground equipment.

In the above-ground equipment, converting means is connected with thetransmitter means in the downhole tool for effecting an analogindication of the temperature encountered in the wellbore in response tothe received signals. As illustrated in FIG. I, the frequency-typesignals are sent through coupling capacitor 61 and via conductor 63 topulse shaper 65 for accurate pulse control and, thence, via conductor 67to integrator and amplifier 69. In the integrator, the frequencytypesignals are integrated and amplified to efl'ect an analog indication ofthe temperature in the borehole in response to the signals. The analogindication is sent over conductor 71 to recorder 73 on which a chart isbeing moved in response to depth signals from the depth functiongenerating means 21 via conductor 75. If desired, an indicator can beconnected with conductor 77 to eflect a direct indication of thetemperature being logged without having to advert to the chart onrecorder 73. The analog indication of temperature from integrator andamplifier 69 is also sent via conductor 77 to differentiator andamplifier 79. The differentiated and amplified analog signal fromdifferentiator and amplifier 79 is also sent to recorder 73 viaconductor 81.

Conventional integrating circuits and conventional differentiatingcircuits can be employed for integrating the frequency-type signals andfor differentiating the resulting analog signal. As indicated before,the analog signal afford: an indicia of the temperature actuallymeasured at a given depth in the borehole. Accordingly, it changesrelatively little, and minute changes might escape detection. On theother hand, a differential of the analog signal indicates a change inthe temperature gradient and is very useful in delineating temperatureanomalies, although it is not of value for determining the actual, orabsolute temperature measured in the borehole. Expressed otherwise, theanalog signal from integrator and amplifier 69 affords a good indicationof the actual temperature being measured whereas the difi'erentiatedsignal is too unstable to be very useful in this respect, but is veryuseful in pointing up changes in the temperature gradient and thusdelineating temperature anomalies.

While, as indicated hereinbefore, batteries can be employed in thedownhole tool, it has been found preferable to supply power at thesurface and generate power of the proper polarities for the respectivepower sources downhole. As illustrated in FIG. I, a DC power source 83is connected downstream of coupling capacitor 61 to facilitate supplyingDC power downhole. It has been found that a positive volts can besupplied and enable obtaining the requisite AC and DC power in thedownhole tool via appropriate power supply means 37 in power supplysection.

It has been found, for example, that a minus 24 volts DC may begenerated by DC-DC converter in the tool and it affords a satisfactorypower source to which to connect the constant current regulator means.This enables satisfactorily controlling the current through the diodejunction means at a desired current level of about one-half milliampere.

In the power supply section, the output from the DC-DC converter may berectified or respectively tapped and rectified to effect the requisiteplus or minus 15 volts for the operationalamplifier.

In operation, tool 1 l is traversed along the longitudinal axis ofborehole 13. Temperature sensor 27 will effect a voltage on conductor59. As indicated, the voltage may start out about a minus 5.6 volts for8 diode junctions in series at surface temperatures and decreaselinearly with increasing temperature to a voltage in the range of abouta minus 3 volts at temperatures of about 350 F. The operationalamplifier amplifies the signal to form an output signal that is afunction of the voltage and consequently of the temperature. Responsiveto the output signal from the operational amplifier, the unijunctiontransistor oscillates at a given frequency for each magnitude outputsignal. The frequency-type signal, consisting of at least pulses of agiven polarity, are transmitted uphole by a transmitter means 35 in theoutput section. The collar locator and amplifier in collar locatorsection 39 may be employed, if desired, to ensure greater accuracy inmeasuring the depth within the borehole and as a supplemental check onthe depth measuring means. In any event, the frequency-type signal istransmitted over a single conductor in cable 17, the armor of whichserves as ground to slip rings and brushes 25 on cable drum 23.

As indicated before, power is supplied from power supply 83 throughcable 17 to the power section where it is appropriately tapped togenerate the requisite voltages for use in downhole tool 11.

The frequency-type signal is sent by way of coupling capacitor 61, whichblocks the direct current power from power source 83, to pulse shaper65. Pulse shaper 65 effects uniform pulses which are integrated byintegrator and amplifier 69. The resulting analog signal is sent torecorder 73 to record with respect to depth the temperature measured inthe borehole. The analog signal is also differentiated by differentiatorand amplifier 79 and the resulting differentiated analog signal recordedvia a pen on recorder 73 to delineate temperature anomalies, or changesin the temperature gradient.

Thus, it can be seen that the invention provides a highly accuratetemperature sensor that has a linear output over the range oftemperatures normally encountered in boreholes penetrating subterraneanformations, yet has a negative temperature coefficient so it is readilyemployed in borehole apparatus; and, consequently, obviates thedisadvantages of the prior art approaches to measuring temperature in aborehole.

Although the invention has been described with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that numerous changes in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention.

What is claimed is:

l. A method of measuring temperature in a borehole penetratingsubterranean formations comprising:

a. flowing a constant current through a forward biased diode junctionmeans having a substantially linearly varying voltage dro at a constantcurrent within the temperature range of 60-350 F;

. moving said diode junction means to a plurality of depths in saidborehole and generating and transmitting to the surface signals that area function of the voltage drop across said diode junction means at saidconstant current; and

c. at the surface converting said signals to a second analog signal thatis a function of the temperature at the respective depth in the boreholeand differentiating said second analog signal to produce an indicationof change in the temperature gradient along said borehole, and recordingwith respect to said depth in said borehole said second analog signalaffording indicia of absolute temperature and the differential of saidsecond analog signal indicating a change in temperature gradient.

2. The method of claim 1 wherein said diode junction means comprise aplurality of diode junctions employed in series to increase sensitivityof the temperature sensor thereby effected.

3. In apparatus for measuring temperature and change in temperature in aborehole penetrating subterranean formations by employing above-groundequipment; a downhole tool suspended on a cable within said borehole formovement along the longitudinal axis of said borehole; and depthmeasuring means for determining the depth of said tool in said boreholethe improvement comprising:

a. diode unction means for meastu'lng temperature, said diode junctionmeans being disposed in said downhole tool and having a substantiallylinearly varying voltage drop at a constant current within thetemperature range of 60-350 F;

b. a constant current regulator means and a power supply means formaintaining a constant current flowing through said diode junctionmeans, said constant current regulator means and said power supply meansbeing serially connected in the forward biased direction with said diodejunction means;

c. signal generating means for generating signals that are a function ofthe voltage present at the juncture of said diode junction means andsaid constant current regulator means, said signal generating meansbeing connected with said diode junction means;

d. transmitter means for transmitting said signals to the above-groundequipment, said transmitter means being connected with said signalgenerating means and with the conductor means in said cable foreffecting an electrically continuous path;

. converting means for effecting an analog indication of saidtemperature in response to said signals, said converting means beingdisposed in said above-ground equipment and connected with saidtransmitter means;

f. differentiator means for difierentiating said analog indication oftemperature to effect an indication of temperature anomalies along thelongitudinal axis of said borehole, said difierentiator means beingconnected with said converting means; and

g. recorder means for recording with respect to depth said analogindication of temperature and said indication of temperature anomalies,said recorder means being connected with said depth measuring means,with said converting means and with said difierentiator means.

4. The apparatus of claim 3 wherein said diode junction means comprise aplurality of diode junctions employed in series to increase sensitivityof the temperature sensor thereby efiected.

5. The apparatus of claim 3 wherein an amplifier means is seriallyconnected with said diode junction means and with said signal generatingmeans for generating and supplying to said signal generating means anoutput signal that is a function of the voltage present at said junctureof said diode junction means and said constant current regulator means.

6. The apparatus of claim 5 wherein said diode junction means comprise aplurality of diode junctions employed in series to increase sensitivityof the temperature sensor thereby effected.

1. A method of measuring temperature in a borehole penetratingsubterranean formations comprising: a. flowing a constant currentthrough a forward biased diode junction means having a substantiallylinearly varying voltage drop at a constant current within thetemperature range of 60*350* F; b. moving said diode junction means to aplurality of depths in said borehole and generating and transmitting tothe surface signals that are a function of the voltage drop across saiddiode junction means at said constant current; and c. at the surfaceconverting said signals to a second analog signal that is a function ofthe temperature at the respective depth in the borehole anddifferentiating said second analog signal to produce an indication ofchange in the temperature gradient along said borehole, and recordingwith respect to said depth in said borehole said second analog signalaffording indicia of absolute temperature and the differential of saidsecond analog signal indicating a change in temperature gradient.
 2. Themethod of claim 1 wherein said diode junction means comprise a pluralityof diode junctions employed in series to increase sensitivity of thetemperature sensor thereby effected.
 3. In apparatus for measuringtemperature and change in temperature in a borehole penetratingsubterranean formations by employing above-ground equipment; a downholetool suspended on a cable within said borehole for movement along thelongitudinal axis of said borehole; and depth measuring means fordetermining the depth of said tool in said borehole, the improvementcomprising: a. diode junction means for measuring temperature, saiddiode junction means being disposed in said downhole tool and having asubstantially linearly varying voltage drop at a constant current withinthe temperature range of 60*-350* F; b. a constant current regulatormeans and a power supply means for maintaining a constant currentflowing through said diode junction means, said constant currentregulator means and said power supply means being serially connected inthe forward biased direction with said diode junction means; c. signalgenerating means for generating signals that are a function of thevoltage present at the juncture of said diode junction means and saidconstant current regulator means, said signal generating means beingconnected with said diode junction means; d. transmitter means fortransmitting said signals to the above-ground equipment, saidtransmitter means being connected with said signal generating means andwith the conductor means in said cable for effecting an electricallycontinuous path; e. converting means for effecting an analog indicationof said temperature in response to said signals, said converting meansbeing disposed in said above-ground equipment and connected with saidtransmitter means; f. differentiator means for differentiating saidanalog indication of temperature to effect an indication of temperatureanomalies along the longitudinal axis of said borehole, saiddifferentiator means being connected with said converting means; and g.recorder means for recording with respect to depth said analogindication of temperature and said indication of temperature anomAlies,said recorder means being connected with said depth measuring means,with said converting means and with said differentiator means.
 4. Theapparatus of claim 3 wherein said diode junction means comprise aplurality of diode junctions employed in series to increase sensitivityof the temperature sensor thereby effected.
 5. The apparatus of claim 3wherein an amplifier means is serially connected with said diodejunction means and with said signal generating means for generating andsupplying to said signal generating means an output signal that is afunction of the voltage present at said juncture of said diode junctionmeans and said constant current regulator means.
 6. The apparatus ofclaim 5 wherein said diode junction means comprise a plurality of diodejunctions employed in series to increase sensitivity of the temperaturesensor thereby effected.